Score in Might & Magic is measured as 1000 x Fame / Time, where time is the number of days completed. With respect to score, a new day starts at 9:00 am. Since Fame is calculated as the sum of the experience (kills and completed quests) of all four characters, which can be influenced by other factors than merely good gameplay, a few groundrules are necessary to make scores compatible. The following "cheats" are out.

[1] Respawning (Rockham, Silver Helm Outpost, The Hive)
[2] Reanimating (Dark spell)
[3] Repeated kills (by paralysis, or if there's no room for the body to drop down)
[4] The learning skill (another bug, it should affect experience but not fame)
[5] Hiring experience-boosting NPC's (same deal)
[6] Experience for money from one Enrothian well (especially when combined with:)
[7] Infinite money from the NWC dungeon

I do also rule out any other gains from the NWC dungeon, such as maximum stats and lots of potions, all of which would save a lot of time.

Let's see, how high scores can become if the ground rules are followed.
The following table shows how much fame is involved with the quests in Might & Magic VI.

Quest	From	Fame
Kriegspire		400
Baa Membership	Almighty Head of Baa	200
Jeweled Egg	Emil Lime	200
Blackshire		520
Pearl of Putrescence	Ghost of Balthazar	20
Pearl of Purity	Ghost of Balthazar	40
Werewolf's Altar	Maria Trepan	80
Emmanuel	Joanna Cravitz	80
Statuettes	Twillen	300
Frozen Highlands		720
Free Haven Temple (Priest)	Anthony Stone	60
Sacred Chalice (High Priest)	Anthony Stone	120
Dragon Tower Keys (Battle Mage)	Erik Von Stromgard	60
Dragon Towers (Warrior Mage)	Erik Von Stromgard	160
Prince of Thieves	Anthony Stone	120
Winter	Erik Von Stromgard	200
Free Haven		4320
Sherell Ivanaveh	Carlo Tormini	40
Mordred	Zoltan Phelps	40
Ethric's Skull	Gabriel Cartman	60
Nomination (Cavalier)	Osric Temper	60
Warlord (Champion)	Osric Temper	160
Devil's Outpost	Osric Temper	160
Memory Crystal Alpha	Oracle of Enroth	400
Memory Crystal Beta	Oracle of Enroth	400
Memory Crystal Delta	Oracle of Enroth	400
Memory Crystal Epsilon	Oracle of Enroth	400
Control Cube	Melian	2000
Arcane Magic	Melian	200
Mire of the Damned		280
Snergle's Axe	Avinryl Smythers	80
Book of Liches	Terry Ros	200
Silver Cove		(220+) 200
Ankh	Loretta Fleise	40
Monolith	Eleanor Vanderbilt	60
Sun Ceremony (Great Druid)	Loretta Fleise	(60)
Moon Ceremony (Arch Druid)	Loretta Fleise	(160)
Price Fixing	Loretta Fleise	100
Bootleg Bay		80
Evil Crystal	Winston Schezar	40
Unward doors	Fire Lord	40
Ironfist		(30+) 392
Sixth Letter to Wilbur Humphrey	Falagar	12
Harp	Andrew Bester	40
Entertainment	Nicolai	(30)
Damsel in distress (Crusader)	Wilbur Humphrey	60
Longfang Witherhide (Hero)	Wilbur Humphrey	120
Lord Kilburn	Wilbur Humphrey	160
Cure for Silvertongue	Wilbur Humphrey	-
Letter to Council	Wilbur Humphrey	-
Third Eye	Nicolai	-
Mist		440
Enemy List	Charles D'Sorpigal	60
Fountain of Magic (Wizard)	Albert Newton	60
Crystal of Terrax (Arch Mage)	Albert Newton	120
Hourglass of Time	Albert Newton	200
New Sorpigal		133
Letter to Andover Potbello	Falagar	1
Angela	Violet Dawson	4
Goblinwatch Code	Janice	8
Candelabra	Andover Potbello	8
Spider Queen	Buford T. Allman	12
Shadowguild Proof	Frank Fairchild	20
Sharry Carnegie	Frank Fairchild	40
Chime of Harmony	Janice	40
Total of all quests		(250+) 7485

Additional fame comes from killing foes, including any Arena fights, and even from killing friends. There are a few special inhabitants of Enroth whose demise does not yield any experience points.
The level of the party influences the strength of the opposition, and there is also a random element involved. In a fast game like Xena's party, all inhabitants of Enroth together bring in about 10,000 fame.
However, it is not necessary to kill all the monsters in order to complete the game. The fastest time to finish is, as you can see from Seoman's log, only 5 days, which leads to scores of over 1,000,000.

If the ground rules as stated above are not followed, there is almost no limit to how high scores can become.